The invention relates to composite MCrAlX-based coatings for superalloy substrates.
Turbine manufacturers have for years used MCrAlX coatings to protect the hot-section components of turbines against corrosion and oxidation. (M is iron, cobalt, nickel, or a combination thereof; X is yttrium, hafnium, tantalum, molybdenum, tungsten, rhenium, rhodium, cadmium, indium, titanium, niobium, silicon, boron, carbon, zirconium, cerium, platinum, or a combination thereof.) As turbine efficiency increases with operating temperature, it is desirable to operate at very high firing temperatures. For applications experiencing these extremely high firing temperatures, more aluminum is added to enhance the coating""s protection. However, when the aluminum concentration exceeds 10-13 weight %, the MCrAlX coating tends to become brittle, often causing delamination of the coating from the substrate. It has become common practice to apply a protective aluminide layer containing 25-35 wt. % aluminum over a MCrAlX coating containing 10 wt. % or less aluminum, in order to increase the amount of aluminum available for oxidation resistance, while prevent failure of the coating by delamination. Unfortunately, the aluminide layer itself is subject to brittleness and cracking, and cracks generated in the brittle aluminide layer can penetrate through the underlying MCrAlX layer and into the substrate, shortening the life of the component.
Accordingly, what is needed is a coating that possesses ductility to minimize crack propagation, while still preserving the necessary oxidation resistance conferred by the presence of an adequate amount of aluminum in the coating.
It has been unexpectedly discovered that use of the composite coatings of the present invention, over a superalloy substrate can significantly improve performance of parts fabricated therefrom. These composite MCrAlX coatings are designed to have a high aluminum concentration while retaining desired ductility. These coatings include a MCrAlX phase, and an aluminum-rich phase having an aluminum concentration higher than that of the MCrAlX phase, and including an aluminum diffusion-retarding composition. The aluminum rich phase supplies aluminum to the coating at about the same rate that aluminum is lost through oxidation, without significantly increasing or reducing the concentration of aluminum in the MCrAlX phase of the coating. The result is excellent oxidation resistance, without an increase in brittleness.
In addition, and in contrast to the two-step process for application of aluminized MCrAlX coatings currently applied on many gas turbine components, the one-step process for applying the coatings of the present invention results in process time and cost savings. For example, the cost of the two-step process is estimated at $2,500 per first-stage bucket, if applied on a large industrial gas turbine bucket, or $230,000.00 for one set of 92 first stage buckets. Because the coating of the present invention does not require an aluminization step, production costs are reduced by half, that is, by approximately $1,250 per bucket, or $115,000 for the set. Further savings may be realized from the doubling of the fatigue life of the first stage buckets made of expensive, nickel-based superalloy. Overall, it is estimated that these savings are equivalent to 4.25% in operating efficiencies.
Elimination of the aluminization step also provides an environmental advantage. Each run of the pack cementation aluminization or xe2x80x9cabove-the-packxe2x80x9d aluminization process produces hundreds of pounds of waste powder containing 1-2% hexavalent chromium, a water soluble substance regulated by the EPA. In comparison, the coating of the present invention is applied without the aluminization process, using materials that are not EPA-regulated.
Accordingly, in one aspect, the present invention relates to a high temperature coating including a MCrAlX phase and an aluminum-rich phase, wherein the amount of the MCrAlX phase ranges from 50-90 parts by weight, and the amount of the aluminum-rich phase ranges from 10-50 parts by weight; in particular, the amount of the MCrAlX phase may range from 70-90 parts by weight, and the amount of the aluminum-rich phase ranges from 10-30 parts by weight; more specifically, the amount of the MCrAlX phase may range from 85-90 parts by weight, and the amount of the aluminum-rich phase may range from 10-15 parts by weight. In the context of the present invention, numerical values recited include all values from the lower value to the upper value in increments of one unit provided that there is a separation of at least two units between any lower value and any higher value. As an example, if it is stated that the amount of a component or a value of a process variable such as, for example, temperature, pressure, time and the like is, for example, from 1 to 90, preferably from 20 to 80, more preferably from 30 to 70, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32 etc. are expressly enumerated in this specification. For values which are less than one, one unit is considered to be 0.0001, 0.001, 0.01 or 0.1, as appropriate. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.
In another aspect, the invention relates to a particulate aluminum composite including a core comprising aluminum, and a shell comprising an aluminum diffusion-retarding composition, whereby the diffusion rate of aluminum from the core to an outer surface of the particles is reduced. The amount of the core may range from 20-95 parts by weight, and of the shell from 5-80 parts by weight.
In yet another aspect, the invention relates to a crack-resistant gas turbine component including the high temperature coating composition of the present invention, and a superalloy substrate.